In an electrical generating plant powered by a large steam turbine, the high pressure turbine is constructed to receive steam through a plurality of arcuately spaced nozzles adjacent the turbine first stage or impulse blading. The steam then flows through the impulse blading to an impulse chamber and through the remaining rows of high pressure reaction blades. The nozzles are segregated into individual groups about the circumference of the impulse blading; and an individual governor valve controls the steam flow through each nozzle group. There are eight governor valves in a typical fossil turbine generating plant, and four governor valves in a typical nuclear turbine generating plant. In turn, steam is directed to the governor valves through one or more throttle or stop valves from the steam source.
When starting up the turbine, it is common practice to operate all the governor valves in a single valve mode to admit the steam in a full 360 degree arc through the nozzles to the impulse blading. This practice, which is termed single valve, or full arc, operation permits the heating of the rotor and rotor blades evenly which minimizes thermal shock. However, when the turbine is "hot" and all the governor valves are admitting the required steam in a partially open position, the efficiency of the plant is considerably reduced because of the pressure drop or throttling action across all the partially open valves. In this situation, the efficiency of the turbine can be increased by transferring to a sequential mode whereby the steam is admitted through a partial arc of fully-open governor valves with the steam flow variations being controlled by one or more of the remaining valves in a sequential manner.
For such sequential or partial arc operation, the governor valves, for control purposes, are segregated into adjacent groups which may be operated in sequence. For example, in a typical fossil fuel plant, there may be four valves operated as a single valve group to control the steam flow at a low demand, and a single valve in each of a second and third sequential groups operated individually to control steam flow at successively higher steam flow demands; and finally the remaining two valves may be operated as a single valve group to control the flow of steam near the maximum steam flow capacity at the plant.
Heretofore, in actual practice, when transferring from single to sequential valve operation a temporary shutdown and/or a recalibration was required, or at the very least a readjustment of the load was necessary after such transfer. In U.S. Pat. No. 3,403,892, a system for transferring between valve modes is proposed wherein the gains and biases of the valves are simultaneously adjusted.
However, it is desirable to be able to transfer back and forth between single and sequential valve modes without any change in load or steam flow to the turbine either during or subsequent to such transfer; and also, it is desirable to be able to accommodate a demand for a change in steam flow, either caused by a change in pressure, or a change in operating requirements of the turbine, for example, during such a transfer, regardless of the actual position of the valves at the time of such change.